Indium tin oxide (ITO) is commonly used as a hole-injecting electrode in organic electroluminescent devices due to its transparency and good electrical conductivity. Various solutions of acid and base have been used to modify the work function of ITO. Depending on the solution, a negative or positive work function shift with respect to that of the standard ITO has been obtained. We have investigated the interface formation between treated ITO and N,N'-bis-(1- naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB), an organic materials often used as hole transport layer in OLEDs, using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The barrier for hole injection, which is defined as the energy difference between the Fermi level and the highest occupied molecular orbital of NPB, decreases from base-, to standard, to acid-treated ITO. No significant reactions is observed for NPB deposited on standard ITO. For the acid- treated ITO substrate, the XPS results point to the reaction between the proton of the adsorbed acid layer and the NPB nitrogen. XPS results also suggest that at low NPB coverage, islanding is occurred at the standard ITO surface.